mod_delegation: use clean_xmlns to remove jabber:client namespace from node:
for the same reason as in mod_privilege, `jabber:client` namespace is removed with the
clean_xmlns method coming from there.
Furthermore, the forwarded <iq> stanza use the `jabber:client` xmlns while the stanza may
come from a component with e.g. `jabber:component:accept` xmlns, this can lead to
inconsistencies between the <iq> stanza and children (like <error> element).